Still not enough entropy on Travis CI, let's see if we can work around that

I tried to fix this using rng-tools in 3c372c3097f77fa4f6d8ff7c67a8e18
but that didn't work out due to the way OpenVZ works. This commit
introduces a more general approach that will hopefully work
on OpenVZ and other virtualized environments, we'll see...
Try to restore Python 3.x compatibility*

* https://travis-ci.org/xolox/python-deb-pkg-tools/jobs/33995580